1. Field of the Invention
The present invention relates to a semiconductor integrated circuit, and more particularly, to a semiconductor integrated circuit including a noise cancelling circuit.
2. Description of Related Art
In recent years, the technology of semiconductor integrated circuit (hereinafter, referred to as “LSI”) has achieved a rapid development for high operating speed and high packing density (high integration). A power supply voltage has been increasingly reduced in order to achieve the high operating speed, with the result that an effect of noise, which is superimposed on an internal power supply, on characteristics such as jitter has been increased. In view of this, there is a demand for a reduction in power supply noise generated in an LSI.
To reduce the power supply noise, a noise cancelling circuit as disclosed in Japanese Unexamined Patent Application Publication No. 10-126237 has been proposed. As shown in FIG. 9, the noise cancelling circuit according to the prior art includes a simultaneous-switching-noise cancelling circuit 93 to cancel switching noise generated by an output stage 92 including a P-channel transistor and an N-channel transistor which are connected between a power supply and a GND.
The simultaneous-switching-noise cancelling circuit 93 includes a GND-side simultaneous-switching-noise cancelling circuit, which is provided so as to cancel the GND-side simultaneous switching noise, and a VCC-side simultaneous-switching-noise cancelling circuit, which is provided so as to cancel the VCC-side simultaneous switching noise.
The GND-side simultaneous-switching-noise cancelling circuit includes: a P-channel transistor having a gate connected to a node D; a diode provided between the power supply provided in the LSI and the node D; a capacitor provided between an input terminal A′ and the node D; and a resistor element provided between the node D and the GND provided in the LSI. Likewise, the VCC-side simultaneous-switching-noise cancelling circuit includes: an N-channel transistor having a gate connected to a node F; a resistor element connected between the power supply provided in the LSI and the node D; a capacitor provided between the input terminal A′ and the node D; and a diode provided between the node F and the GND provided in the LSI.
Operations of the simultaneous-switching-noise cancelling circuit 93 configured as described above will be described with reference to FIG. 10.
First, signals shown in FIG. 10 are described. A signal “A′” corresponds to a voltage of an input signal input to the output stage 92, and is input to each of a gate of the P-channel transistor and a gate of the N-channel transistor, which constitute the output stage 92, as a voltage changing from “L” to “H” or a voltage changing from “H” to “L”. A signal “A” corresponds to a voltage output from the output stage 92 to a signal terminal in accordance with an input voltage of the input signal “A′”. A signal “B” corresponds to a current flowing from the output stage to a GND terminal, and also corresponds to a current caused to flow due to an L-component of a lead of the GND terminal. A signal “C” corresponds to a current flowing from a power supply terminal to the output stage. A signal “D” corresponds to a voltage at the node D, that is, a voltage applied to the gate of the P-ch FET forming the circuit that cancels the simultaneous switching noise generated due to the L-component of the lead of the GND terminal. A signal “E” corresponds to a current flowing to the noise cancelling circuit of the P-ch FET, that is, a current caused to flow when the signal “D” is applied to the gate. A signal “F” corresponds to a voltage at the node F, that is, a voltage applied to the gate of the N-ch FET forming the circuit that cancels the simultaneous switching noise generated due to the L-component of the lead of the GND terminal. A signal “G” corresponds to a current flowing to the noise cancelling circuit of the N-ch FET, that is, a current caused to flow when the signal “F” is applied to the gate. A signal “H” corresponds to a total amount of current flowing to the power supply terminal, that is, a current represented as H=C+G, which is adjusted to become “0” in this case. A signal “J” corresponds to a total amount of current flowing to the GND terminal, that is, a current represented as J=B+E, which is adjusted to become “0” in this case.
As described above, when each power supply terminal and each GND terminal are provided to the simultaneous-switching-noise cancelling circuit, the following effects can be obtained.    (1) When the signal “A′” is changed from “H” to “L” and when the output voltage from the output stage 92 is changed from “L” to “H”, the simultaneous switching noise generated due to the lead component of the power supply terminal can be cancelled, and when a cancel current is caused to flow to the power supply terminal, the interference caused on the GND terminal side due to the cancel current is eliminated, which contributes to a stable operation.    (2) When the signal “A′” is changed from “L” to “H” and when the output voltage from the output stage 92 is changed from “H” to “L”, the simultaneous switching noise generated due to the lead component of the GND terminal can be cancelled, and when the cancel current is caused to flow to the GND terminal, the interference caused on the power supply terminal side due to the cancel current is eliminated, which contributes to a stable operation.
As described above, the power-supply-noise cancelling circuit according to the prior art determines the operation of the simultaneous-switching-noise cancelling circuit 93, that is, determines a timing at which the noise cancellation is carried out, in accordance with the change of the input signal input to the output stage 92. As a result, even when an external load connected to the output of the output stage 92 is varied, the timing for the current to cancel the noise is not changed.
Accordingly, when the external load is varied in the power-supply-noise cancelling circuit according to the prior art, the output voltage “A”, the current “B” flowing from the output stage to the GND terminal, and the current “C” flowing from the power supply terminal to the output stage are varied depending on the magnitude of the external load. Meanwhile, the current “E” flowing to the P-channel transistor of the GND-side simultaneous-switching-noise cancelling circuit, and the current “G” flowing to the N-channel transistor of the VCC-side simultaneous-switching-noise cancelling circuit are not varied irrespective of the magnitude of the external load. Accordingly, when the load is large, B>E and C>G are satisfied. Further, when the load is small, B<E and C<G are satisfied.
As a result, the total amount “H” of currents flowing to the power supply terminal and the total amount “J” of currents flowing to the GND terminal are not enough to absorb the fluctuation of the load, which causes a problem in that the current cancellation cannot be satisfactorily carried out.